PACIFIC COAST OLIGOCH^TA. 65 



Body-wall. The various layers ofler nothing of great interest. The longitud- 

 inal muscular la^'cr is slightly thicker at the dorsal part, and so is the part situated 

 between the od and 4th seta". The muscles in this layer do not show any feathery 

 arrangement around a central fiber, but are irregularly distributed. There are no 

 dorsal pores. The body-wall in somites xi and xii istliinner than the anterior somites. 



Arciform muscles. The inner .surface of the body-wall in somites xvii and 

 xviii, and partially also in xvi, are covered with numerous arciform muscles which 

 sti'ctch transversely or diagonall}^ across the body cavity, connecting the region run- 

 ning through the outer couple of set;io with that of the male pore and the inner setjc. 

 The number of muscular bands in these somites is very great and they vary in length 

 and thickness. There is also a slight variation in different specimens, the main fasci- 

 cles being, however, always in the main the same. These muscles are best viewed 

 when seen from above, the body wall being spread open and the sacculated intestine 

 removed. As will be seen from the drawing, which is a careful representation of the 

 principal muscular bands, most of the bands begin or are attached to the body-wall on 

 a line running througli setse 1 and 2 and ending in a line running through setie 

 3 and 4. But there are some which begin more ventrally and end more dorsally 

 than cither couple of settc. In somite xviii most of the fascicles run in right angle 

 to the median line, but the most posterior one as well as one or two more run diag- 

 onally backwards. In somite xvii there is one large group of fascicles beginning 

 around the male pore and running transversely sideways, while another group of fas- 

 cicles run diagonally forward connecting the male region with the anterior part of 

 the somite. There are also longer fascicles anterior of the male pore which run much 

 further sideways than the outer setae. In somite xvi there are but few fascicles, 

 much smaller and shorter. The male i5ore and spermducts are so entii'ely covered 

 up by these muscular fascicles that they can only be seen with the greatest difficulty 

 when viewed on the spread body-wall. In the figure (fig. 7), only the principal mus- 

 cles of one side ai'e drawn. The shaded bands which are seen crossing the median 

 line are slight ridges in the body-wall, connecting the principal muscle fascicles. The 

 objects of these muscles are of course to elevate and depress the male zone. Similar 

 muscles were first described by Benham in Moniligaster, and later by myself in Argi- 

 lophilus. They no doubt exist in most species of 01igoch«ta but are of great interest 

 and value as species characteristics. 



The septa begin between somites v and vi, and gradually increase in size 

 toward somite ix. The most anterior septum, however, is not unusually thick. The 

 following three septa are very thick, that between viii and ix being the thickest. 



Alimentary canal. The buccal cavity is very large and occupies somites i and 

 ii. Pharynx begins in iii or posterior part of ii and extends to the posterior part of 

 iii. It is only developed superiorly and merely attached to the oesophagus in the pos- 

 terior part of ii. Qilsophagus consists of one in the beginning narrow and compara- 

 tively even duct, which in iv rises upwards and then becomes considerably sacculated 



